The HL-2M tokamak is a new medium-sized tokamak at SouthWestern Institute of Physics. Two of its key missions are to achieve 10 keV ion temperature and investigate the behavior of energetic particles relevant to burning plasmas. A 6 MW ion cyclotron range of frequencies (ICRF) heating power is embedded in the next upgrade program of HL-2M. In order to facilitate the engineering design of the ICRF system, this paper analyses the main ICRF heating schemes for HL-2M, in terms of ion heating and energetic ion generation in particular. D(H) minority heating and the 2nd harmonic D will act as the main ion heating schemes, for which the optimal RF frequency range 27–33 MHz, antenna parallel wavenumber k // ∼ 8 m−1 are proposed and strong single pass absorption is expected under typical HL-2M plasma parameters. Full wave simulations carried out via TORIC/steady-state Fokker–Planck quasilinear solver and TRANSP codes suggest that by adopting three ion scheme or synergetic heating on neutral beam injection D ions by the 2nd harmonic D, energetic ions with energy at MeV level can be produced. This study shows that ICRF heating could play significant roles in ion heating, energetic ion generation in HL-2M.

https://iopscience.iop.org/article/10.1088/1741-4326/acc4dc/pdf

ICRF heating schemes for the HL-2M tokamak

Fast ions synergy induced by ion-cyclotron range of frequency (ICRF) and neutral beam injection (NBI) are of interest not only because of their advantage of heating the plasma and drive currents, but also because of their disadvantage of damaging plasma surface components and driving MHD instabilities. In this paper, we calculate the fast ion loss and the deposition distribution of the lost particles on the limiters in EAST under the synergistic effect of the ripple field and collisions with the full-orbit-following simulation program ISSDE for the first time. The previous models to study the NBI fast ion loss by the action of ICRF are relatively simple and consider fewer influencing factors. Most studies on fast ion loss have used toroidal uniform boundaries. In this work, we consider the distribution of ICRF-NBI synergy induced fast ions with different minority H concentrations. After setting the limiter boundary, we consider the prompt fast ion loss caused by the equilibrium field and the fast ion loss caused by the ripple field and collision. Under the action of minority-ion ion-cyclotron resonant heating, the NBI fast ion distribution function has spread in the high-energy part, especially for the minority H concentration of 1%, and the fast ions show each anisotropic distribution near the resonance band on the poloidal dimension. The synergistic loss caused by the ripple field and collision will first be greater than the loss caused by either factor, and then reach a final loss fraction of 3.8%. The heat load power density of the lost fast ions on different limiters is not uniform, as well as on each limiter, which is related to the distance from the limiter to the plasma, the relative position between the limiters and the parallel direction of most fast ions. Once the study of ICRF-NBI synergy induced fast ion loss caused by the action of ripple and collision has been done, we can do optimization in a targeted manner. Such as adding ferromagnetic inserts to reduce the ripple loss and optimizing the limiters’ position to reduce or control the generation of impurities.

https://iopscience.iop.org/article/10.1088/1741-4326/acbdad/pdf

3D simulation of orbit loss and heat load on limiters of ICRF-NBI synergy induced fast ions on EAST

Abstract Efficient ion cyclotron range of frequencies (ICRF) wave heating requires good wave coupling at the plasma edge and good radio frequency power absorption in the plasma core. This study reviews recent progress in improving these two aspects of ICRF heating with the new two-strap antennas through various experiments and simulations on the Experimental Advanced Superconducting Tokamak (EAST). Our study shows that the ICRF coupling can be significantly improved by decreasing the parallel wave number, increasing the local scrape-off layer (SOL) density by midplane gas puffing, and increasing the global SOL density by decreasing the separatrix–antenna distance. It can also be improved by increasing the core plasma density, changing the divertor strike point position, and optimizing the antenna phasing. The core ICRF power absorption can be increased by optimizing the cyclotron resonance position and minority ion concentration and by applying new heating schemes such as three-ion heating. Although some of the methods have been previously studied on other devices, improving ICRF coupling by shifting the divertor strike point was tested on EAST for the first time. Quantitative characterization of these methods and the conclusions drawn from this study can provide important insights for achieving more efficient ICRF heating in current and future fusion machines. 

Recent progress in improvement in ion cyclotron range of frequencies coupling and power absorption with new antennas of Experimental Advanced Superconducting Tokamak (EAST)


 This study investigates the single-pass absorption (SPA) of ion cyclotron range of frequency (ICRF) heating in hydrogen plasma of the EXL-50U spherical tokamak, which is an upgraded device of the EXL-50 with a central solenoid and a stronger magnetic field. The reliability of the kinetic dispersion equation is confirmed by the one-dimensional full-wave code, and the applicability of Porkolab’s simplified theoretical SPA model is discussed based on the kinetic dispersion equation. Simulations are conducted to investigate the heating effects of the fundamental and second harmonic frequencies. The results indicate that with the design parameters of the EXL-50U device, the SPA for second harmonic heating is 63%, while the SPA for fundamental heating is 13%. Additionally, the optimal injection frequencies are 23 MHz at 0.9 T and 31 MHz at 1.2 T. The wave vector of the antenna parallel to the magnetic field, with a value of k∥ = 7.5 m−1, falls within the optimal heating region. Simulations reveal that the ICRF heating system can play an important role in the ion heating of the EXL-50U.

Simulation of ion cyclotron wave heating in the EXL-50U spherical tokamak based on dispersion relations


 Two new ICRF antennas operating in the ion cyclotron radio frequency (ICRF) range have been developed for EAST to overcome the low coupling problem of the original antennas. The original ICRF antennas were limited in their power capacity due to insufficient coupling. The new antenna design takes into account both wave coupling and absorption processes through comprehensive wave coupling and absorption codes, with the dominant parallel wave number k_∥ of 7.5 m-1 at dipole phasing. Through the use of these new ICRF antennas, we are able to achieve 3.8 MW output power and 360 second operation, respectively. The initial experimental results demonstrate the reliability of the antenna design method.

Physical design and recent experimental results of the new ICRF antenna on EAST

Ion Cyclotron Range of Frequencies (ICRF) heating and Neutral Beam Injection (NBI) can have synergy due to the acceleration of NBI beam ions by ICRF wave fields at their harmonics. To understand the influence of ICRF-NBI synergy on fast ion distribution and plasma performance, dedicated experiments and TRANSP simulations have been carried out on EAST. The simulation results are consistent with the experimental results. They show that the ICRF-NBI synergy not only accelerates the NBI beam ions with energy lower than 80 keV to energy larger than 300 keV, but also generates fusion neutrons with energy larger than 3 MeV. Moreover, ICRF-NBI synergy improves the plasma performance by increasing the poloidal beta, plasma stored energy, core ion temperature, total neutron yield and kinetic pressure. In a typical H-mode plasma with 1.0 MW NBI and 1.5 MW ICRF power, it was observed that ICRF-NBI synergy increases the poloidal beta, plasma stored energy, core ion temperature and neutron yield by ∼35%, 33%, 22% and 80%, respectively. Various parameter scans show that the ICRF-NBI synergetic effects can be enhanced by decreasing the minority ion concentration or the distance between the harmonic resonance and magnetic axis, or by increasing the ICRF heating power or NBI beam energy. Consequently, this leads to a generation of fast ions with higher energy. For instance, the maximum energy of the fast ion tail increases from 300 to 600 keV as n(H) decreases from 5% to 0.1%.

/pdf/influence-of-icrf-nbi-synergy-on-fast-ion-distribution-and-1gfetix1.pdf

Influence of ICRF-NBI synergy on fast ion distribution and plasma performance in second harmonic heating experiments with deuterium NBI at EAST

In tokamak experiments, the BT ramping down can drive off-axis and parallel inductive current. This approach leads to a decrease in q95 and an increase in normalized beta, βN. The off-axis inductive current also broadens the current profile. Typically, the 1.5D transport code is used to simulate the time evolution of the plasma current profile, which is based on the flux-surface-averaged Faraday’s law. The ONETWO code is one of those transport codes. However, this code cannot simulate the situation of the evolving toroidal field BT. This article proposes a new time-dependent model to take into account the BT ramping down situation. A modified formula of flux-surface-averaged Faraday’s law was derived to consider the effect of the BT ramping rate on the current evolution, and it was implemented in the ONETWO code. Then, the modified ONETWO code was used to simulate the BT ramping down experiment on DIII-D. The simulation result of the plasma current evolution with BT ramping down shows a broader current profile with smaller ohmic current induced by the poloidal field, compared to that without BT ramping down.

Simulations of plasma current induced by toroidal field ramping down on tokamaks

The main limiter in EAST was observed to endure a high heat load and was cracked near the midplane at the right side during the plasma operation. To explore the heat load carried by fast ion loss toward the main limiter, the neutral beam injection (NBI) and radio frequency power proportion experiment was conducted in EAST where the plasma stored energy and line integrated density were kept almost constant. The hot spot at the right side of the main limiter was observed to be enhanced by co-current perpendicular (co-perp) NBI. An NBI ion loss simulation was performed in the presence of the toroidal field ripple and the Coulomb collision by using the orbit code GYCAVA and the NBI code TGCO. The result indicates that the NBI ion loss by ripple and collision mainly causes a bright area below the midplane of the right side of the main limiter as observed in the EAST experiment. The peak heat load of lost fast ions generated by co-perp NBI is ∼0.5 MW/m2 as obtained by 1 MW of NBI deposited power and comparable with the heat load carried by fast electrons induced by lower hybrid current drive. In addition, increasing the gap between the separatrix and the first-wall limiters in the simulation is found to reduce this heat load. 

Hot spots of the main limiter induced by fast ions in experimental advanced superconducting tokamak (EAST)

The poloidal width of the filaments induced by the type-I edge localized mode has power dependence in EAST. The poloidal widths of the filaments observed by the high-speed vacuum ultraviolet (VUV) imaging system are proportional to the heating power and the ELM size. To understand this power dependence, the BOUT++ nonlinear simulations have been performed with the reconstructed equilibriums from the experimental measurements in this paper. The synthetic filament structures from BOUT++ nonlinear simulation match the experimental observations by the VUV imaging system. The BOUT++ nonlinear simulations also reproduce the power dependence of the filament widths and the ELM size. The filament width and the ELM size are inversely proportional to the toroidal mode number. The low-n mode has a broader radial and poloidal structure, which causes the larger filament width and ELM size. In the high input power case, the mode spectrum shifts to low-n, a result of increasing peeling drive. Besides, we found the β p in a higher input power case leads to a broader pedestal, expanding the radial mode structure of the peeling-ballooning mode.

Study on filament width of type-I ELM in EAST using VUV imaging system and simulation

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Papers

Influence of ICRF-NBI synergy on fast ion distribution and plasma performance in second harmonic heating experiments with deuterium NBI at EAST

Simulations of plasma current induced by toroidal field ramping down on tokamaks

Hot spots of the main limiter induced by fast ions in experimental advanced superconducting tokamak (EAST)

Study on filament width of type-I ELM in EAST using VUV imaging system and simulation